This invention relates to an improved modulating plug type valve for controlling the flow of fluids. More particularly, the invention relates to an arrangement for mounting the plug and disc of a plug valve onto the valve stem for allowing a higher degree of accuracy in the modulation of the fluid flow.
Plug and disc type valves are known in the prior art. Such valves comprise a disc cooperating with the valve seat for cutting off fluid flow from the inlet to the outlet openings of the valve. This disc is movably attached to the valve stem for allowing the disc to angularly align with the valve seat.
Rigidly affixed to the disc is a plug which, when the disc is seated against the valve seat, extends into the valve seat. The plug is contoured such that, as the valve disc lifts off the valve seat to permit flow from the inlet opening to the outlet opening, the amount of flow is regulated by an amount determined by the amount of separation between the disc and seat. The surface of the plug must be characterized, i.e. contoured, to compensate for the non-linearity of the operator of the valve. Since the plug is rigidly fixed to the valve stem and disc assembly, the plug must be sized to provide enough tolerance between the plug and valve seat to allow the plug to easily move within the valve seat but yet allow for modulation of the flow from inlet to outlet.
This tolerance must necessarily be large since the disc and plug are rigidly secured together and any angular movement of the disc to seat against a misaligned valve seat also results in angular movement of the plug. Because this tolerance must be sufficiently large for accomplishing these purposes, the initial separation of the disc from the valve seat allows a surge of fluid to flow from inlet to outlet. Thus, the modulating effect for small openings of the valve is minimal. Furthermore, the prior art arrangement of attaching the disc and plug to the valve stem is costly,.
The prior art also has the disadvantage that the forces imposed upon the plug and disc assembly are applied through the alignment mechanism. Heavy loads and friction on the plug and disc assembly and alignment mechanism, therefore, impede the alignment of the plug within the port defined by the valve seat creating excessive loads and wear on the valve stem.